DESCRIPTION

BN_generate_prime() generates a pseudo-random prime number of num
bits.
If ret is not NULL, it will be used to store the number.

If callback is not NULL, it is called as follows:

•

callback(0, i, cb_arg) is called after generating the i-th
potential prime number.

•

While the number is being tested for primality, callback(1, j,
cb_arg) is called as described below.

•

When a prime has been found, callback(2, i, cb_arg) is called.

The prime may have to fulfill additional requirements for use in
Diffie-Hellman key exchange:

If add is not NULL, the prime will fulfill the condition p % add
== rem (p % add == 1 if rem == NULL) in order to suit a given
generator.

If safe is true, it will be a safe prime (i.e. a prime p so
that (p-1)/2 is also prime).

The PRNG must be seeded prior to calling BN_generate_prime().
The prime number generation has a negligible error probability.

BN_is_prime() and BN_is_prime_fasttest() test if the number a is
prime. The following tests are performed until one of them shows that
a is composite; if a passes all these tests, it is considered
prime.

BN_is_prime_fasttest(), when called with do_trial_division == 1,
first attempts trial division by a number of small primes;
if no divisors are found by this test and callback is not NULL,
callback(1, -1, cb_arg) is called.
If do_trial_division == 0, this test is skipped.

Both BN_is_prime() and BN_is_prime_fasttest() perform a Miller-Rabin
probabilistic primality test with checks iterations. If
checks == BN_prime_checks, a number of iterations is used that
yields a false positive rate of at most 2^-80 for random input.

If callback is not NULL, callback(1, j, cb_arg) is called
after the j-th iteration (j = 0, 1, ...). ctx is a
pre-allocated BN_CTX (to save the overhead of allocating and
freeing the structure in a loop), or NULL.

RETURN VALUES

BN_generate_prime() returns the prime number on success, NULL otherwise.

BN_is_prime() returns 0 if the number is composite, 1 if it is
prime with an error probability of less than 0.25^checks, and
-1 on error.

SEE ALSO

HISTORY

The cb_arg arguments to BN_generate_prime() and to BN_is_prime()
were added in SSLeay 0.9.0. The ret argument to BN_generate_prime()
was added in SSLeay 0.9.1.
BN_is_prime_fasttest() was added in OpenSSL 0.9.5.